Dinuclear clusters bond order

Most of the compounds with metal-metal double bonding do not arise directly from the analogue of higher bond order discussed above. Thus for instance in the chemistry of rhenium compounds the best known and also most interesting species are the Re (III) trinuclear clusters that will be discussed in the next chapter. In this sense the chemistry of compounds with metal-metal double bonds appears as a transition from the chemistry of dinuclear metal compounds to that of higher metal aggregates. 

The factors which lead to the formation of dinuclear compounds containing M-M bonds of multiple order n rather than to the formation of polynuclear or cluster compounds in which the metal atoms form n o-bonds with each other are not well understood. The size of the ligands... 

In order to facilitate the discussion of some aspects of cluster chemistry-especially those related with bonding and its relationship with chemical and structural properties of the compounds-it is convenient to review briefly the fundaments of current bonding concepts. Some germane aspects of the atomic and molecular structure will be therefore summarized in the next sections. As an application of such concepts and also as an introduction to metal clusters, the discussion of some aspects of the metal-metal multiple bonding in dinuclear compounds is also included in this Chapter. 

---